Scanned-Spot-Array Optical Lithography is a maskless lithographic printing method in which an array of diffraction-limited focused-radiation spots is raster-scanned over a printing surface (a photosensitive optical recording medium) to synthesize a high-resolution recorded image. The spots may be individually modulated by a spatial light modulator. Alternatively, the spots are not individually modulated, but are collectively modulated by a single modulator.
Maskless EUV systems employing scanned-spot arrays are described in U.S. Pat. Nos. 6,498,685 and 7,116,405. In these systems, the spots are generated by very small EUV-transmitting microlenses of comparatively high numerical aperture (NA) in close proximity to the printing surface. Such systems would have limited practicality due to the low optical efficiency, nonuniform transmittance, and chromatic dispersion of high-NA EUV microlenses, as well as the complication of maintaining accurate positioning of a large-area microlens array very close to the printing surface.
The '843 application discloses an alternative scanned-spot-array system in which the spots are generated at the object surface of a projection lens, and are imaged by the lens onto a printing surface at reduced magnification. This makes it possible to use comparatively simple, low-NA microlenses for spot generation. Moreover, the microlenses can be configured to offset and neutralize geometric optical aberrations in the projection lens. But the utility of this type of system for EUV lithography would still be limited by the inefficiency, nonuniform transmittance, and chromatic dispersion of EUV microlenses. The '843 application describes an EUV embodiment using spatially-filtered, zone-plate microlenses, which would partially alleviate but not fully overcome these limitations. Reflective microlenses (i.e., micromirrors) do not suffer these limitations, but the micromirror embodiment illustrated in '843 FIG. 32 is not configured for EUV lithography. The '843 application further notes that “Micromirrors have the disadvantage that the focused spots cannot be spatially filtered . . . because such [spatial filter] apertures would interfere with the illumination optical path.”